Memory element and memory device

ABSTRACT

There is provided a memory element including a memory layer that has magnetization perpendicular to a film face; a magnetization-fixed layer that has magnetization that is perpendicular to the film face; and an insulating layer that is provided between the memory layer and the magnetization-fixed layer, wherein an electron that is spin-polarized is injected in a lamination direction of a layered structure, and thereby the magnetization direction of the memory layer varies and a recording of information is performed, a magnitude of an effective diamagnetic field which the memory layer receives is smaller than a saturated magnetization amount of the memory layer, the insulating layer is formed of an oxide film, and the memory layer is formed of Co—Fe—B, a concentration of B is low in the vicinity of an interface with the insulating layer, and the concentration of B increases as it recedes from the insulating layer.

CROSS REFERENCES TO RELATED APPLICATIONS

The present application claims priority to Japanese Priority PatentApplication JP 2010-199716 filed in the Japan Patent Office on Sep. 7,2010, the entire contents of which are hereby incorporated by reference.

BACKGROUND

The present application relates to a memory element that includes amemory layer that stores the magnetization state of a ferromagneticlayer as information and a magnetization-fixed layer in which amagnetization direction is fixed, and that changes the magnetizationdirection of the memory layer by flowing a current, and a memory devicehaving the memory element.

In an information device such as a computer, a highly dense DRAM thatoperates at a high speed has been widely used as a random access memory.

However, the DRAM is a volatile memory in which information is erasedwhen power is turned off, such that a non-volatile memory in which theinformation is not erased is desirable.

In addition, as a candidate for the non-volatile memory, a magneticrandom access memory (MRAM) in which the information is recorded bymagnetization of a magnetic material has attracted attention andtherefore has been developed.

The MRAM makes a current flow to two kinds of address interconnects (aword line and a bit line) that are substantially perpendicular to eachother, respectively, and inverts the magnetization of a magnetic layerof a magnetic memory element, which is located at an intersection of theaddress interconnects, of the memory device using a current magneticfield generated from each of the address interconnects, and therebyperforms the recording of information.

A schematic diagram (perspective view) of a general MRAM is shown inFIG. 8.

A drain region 108, a source region 107, and a gate electrode 101, whichmake up a selection transistor that selects each memory cell, are formedat portions separated by an element separation layer 102 of asemiconductor substrate 110 such as a silicon substrate, respectively.

In addition, a word line 105 extending in the front-back direction inthe drawing are provided at an upper side of the gate electrode 101.

The drain region 108 is formed commonly to left and right selectiontransistors in the drawing, and an interconnect 109 is connected to thedrain region 108.

In addition, magnetic memory elements 103, each having a memory layerwhose magnetization direction is inverted, are disposed between the wordline 105 and bit lines 106 that are disposed at an upper side inrelation to the word line 105 and extend in the left-right direction.These magnetic memory elements 103 are configured, for example, by amagnetic tunnel junction element (MTJ element).

In addition, the magnetic memory elements 103 are electrically connectedto the source region 107 through a horizontal bypass line 111 and avertical contact layer 104.

When a current is made to flow to the word line 105 and the bit lines106, a current magnetic field is applied to the magnetic memory element103 and thereby the magnetization direction of the memory layer of themagnetic memory element 103 is inverted, and therefore it is possible toperform the recording of information.

In addition, in regard to a magnetic memory such as the MRAM, it isnecessary for the magnetic layer (memory layer) in which the informationis recorded to have a constant coercive force in order to stably retainthe recorded information.

On the other hand, it is necessary to make a certain amount of currentflow to the address interconnect in order to rewrite the recordedinformation.

However, along with miniaturization of the element making up the MRAM,the address interconnect becomes thin, such that it is difficult to flowa sufficient current.

Therefore, as a configuration capable of realizing the magnetizationinversion with a relatively small current, a memory having aconfiguration using magnetization inversion by spin injection hasattracted attention (for example, refer to Japanese Unexamined PatentApplication Publication Nos. 2003-17782 and 2008-227388, and aspecification of U.S. Pat. No. 6,256,223, PHYs. Rev. B, 54.9353 (1996),and J. Magn. Mat., 159, L1 (1996).

Magnetization inversion by the spin injection means that a spinpolarized electron after passing through a magnetic material is injectedto the other magnetic material, and thereby magnetization inversion iscaused in the other magnetic material.

For example, when a current is made to flow to a giant magnetoresistiveeffect element (GMR element) or a magnetic tunnel junction element (MTJelement) in a direction perpendicular to a film face, the magnetizationdirection of at least a part of the magnetic layer of this element maybe inverted.

In addition, magnetization inversion by spin injection has an advantagein that even when the element becomes minute, it is possible to realizethe magnetization inversion without increasing the current.

A schematic diagram of the memory device having a configuration usingmagnetization inversion by the above-described spin injection is shownin FIGS. 9 and 10. FIG. 9 shows a perspective view, and FIG. 10 shows across-sectional view.

A drain region 58, a source region 57, and a gate electrode 51 that makeup a selection transistor for the selection of each memory cell areformed, respectively, in a semiconductor substrate 60 such as a siliconsubstrate, at portions isolated by an element isolation layer 52. Amongthem, the gate electrode 51 also functions as a word line extending inthe front-back direction in FIG. 9.

The drain region 58 is formed commonly to left and right selectiontransistors in FIG. 9, and an interconnect 59 is connected to the drainregion 58.

A memory element 53 having a memory layer in which a magnetizationdirection is inverted by spin injection is disposed between the sourceregion 57 and bit lines 56 that are disposed in an upper side of thesource region 57 and extend in the left-right direction in FIG. 9.

This memory element 53 is configured by, for example, a magnetic tunneljunction element (MTJ element). The memory element 53 has two magneticlayers 61 and 62. In the two magnetic layers 61 and 62, one sidemagnetic layer is set as a magnetization-fixed layer in which themagnetization direction is fixed, and the other side magnetic layer isset as a magnetization-free layer in which that magnetization directionvaries, that is, a memory layer.

In addition, the memory element 53 is connected to each bit line 56 andthe source region 57 through the upper and lower contact layers 54,respectively. In this manner, when a current is made to flow to thememory element 53, the magnetization direction of the memory layer maybe inverted by spin injection.

In the case of the memory device having a configuration using magneticinversion by this spin injection, it is possible to make the structureof the device simple compared to the general MRAM shown in FIG. 8, andtherefore it has a characteristic in that high densification becomespossible.

In addition, when magnetization inversion by spin injection is used,there is an advantage in that even as miniaturization of the elementproceeds, the write current is not increased, compared to the generalMRAM performing magnetization inversion by an external magnetic field.

SUMMARY

However, in the case of the MRAM, a write interconnect (word line or bitline) is provided separately from the memory element, and the writing ofinformation (recording) is performed using a current magnetic fieldgenerated by flowing a current to the write interconnect. Therefore, itis possible to make a sufficient amount of current necessary for thewriting flow to the write interconnect.

On the other hand, in the memory device having a configuration usingmagnetization inversion by spin injection, it is necessary to invert themagnetization direction of the memory layer by performing spin injectionusing a current flowing to the memory element.

Since the writing (recording) of information is performed by directlyflowing a current to the memory element as described above, a memorycell is configured by connecting the memory element to a selectiontransistor to select a memory cell that performs the writing. In thiscase, the current flowing to the memory element is restricted to acurrent magnitude capable of flowing to the selection transistor (asaturation current of the selection transistor).

Therefore, it is necessary to perform writing with a current equal to orless than the saturation current of the selection transistor, andtherefore it is necessary to diminish the current flowing to the memoryelement by improving spin injection efficiency.

In addition, to increase the read-out signal strength, it is necessaryto secure a large magnetoresistance change ratio, and to realize this,it is effective to adopt a configuration where an intermediate layerthat comes into contact with both sides of the memory layer is set as atunnel insulating layer (tunnel barrier layer).

In this way, in a case where the tunnel insulating layer is used as theintermediate layer, the amount of current flowing to the memory elementis restricted to prevent the insulation breakdown of the tunnelinsulating layer. From this viewpoint, it is also necessary to restrictthe current at the time of spin injection.

Since such a current value is proportional to a film thickness of thememory layer and is proportional to the square of the saturationmagnetization of the memory layer, it may be effective to adjust these(film thickness and saturated magnetization) to decrease such a currentvalue (for example, refer to F. J. Albert et al., Appl. Phys. Lett., 77,3809 (2000).

For example, in F. J. Albert et al., Appl. Phys. Lett., 77, 3809 (2000),the fact that when the amount of magnetization (Ms) of the recordingmaterial is decreased, the current value may be diminished is disclosed.

However, on the other hand, if the information written by the current isnot stored, a non-volatile memory is not realized. That is, it isnecessary to secure stability (thermal stability) against the thermalfluctuation of the memory layer.

In the case of the memory element using magnetization inversion by spininjection, since the volume of the memory layer becomes small, simplyconsidered thermal stability tends to decrease, compared to the MRAM inthe related art.

When thermal stability of the memory layer is not secured, the invertedmagnetization direction re-inverts by heating, and this leads to writingerrors.

In addition, in a case where high capacity of the memory element usingmagnetization inversion by spin injection is advanced, the volume of thememory element becomes smaller, such that securing thermal stabilitybecomes an important problem.

Therefore, in regard to the memory element using magnetization inversionby spin injection, thermal stability is a very important characteristic.

Therefore, to realize a memory element having a configuration where themagnetization direction of the memory layer as a memory is inverted byspin injection, it is necessary to diminish the current necessary formagnetization inversion by spin injection to a value equal to or lessthan the saturation current of the transistor, and thereby securingthermal stability for retaining written information reliably.

As described above, to diminish the current necessary for magnetizationinversion by spin injection, diminishing the saturated magnetizationamount Ms of the memory layer, or making the memory layer thin may beconsidered. For example, as is the case with U.S. Pat. No. 7,242,045, itis effective to use a material having a small saturated magnetizationamount Ms as the material for the memory layer. However, in this way, ina case where the material having the small saturated magnetizationamount Ms is simply used, it is difficult to secure thermal stabilityfor reliably retaining information.

Therefore, in this application, it is desirable to provide a memoryelement capable of improving thermal stability without increasing thewrite current, and a memory device with the memory element.

According to an embodiment, there is provided a memory element includinga memory layer that has magnetization perpendicular to a film face andthe magnetization direction thereof varies corresponding to information;a magnetization-fixed layer that has magnetization that is perpendicularto the film face and becomes a reference for the information stored inthe memory layer; and an insulating layer that is provided between thememory layer and the magnetization-fixed layer and is formed of anon-magnetic layer, wherein an electron that is spin-polarized isinjected in a lamination direction of a layered structure having thememory layer, the insulating layer, and the magnetization-fixed layer,and thereby the magnetization direction of the memory layer varies andrecording of information is performed with respect to the memory layer.A magnitude of an effective diamagnetic field which the memory layerreceives is smaller than saturated magnetization amount of the memorylayer. The insulating layer is formed of an oxide film (for example, anMgO film), the memory layer is formed of Co—Fe—B, a concentration of Bis low in the vicinity of the interface with the insulating layer, andthe concentration of B increases as it recedes from the insulatinglayer.

For example, the memory layer may have a laminated structure of aplurality of Co—Fe—B layers, and each of the Co—Fe—B layers may beconfigured in such a manner that a layer that comes into contact withthe insulating layer is set as a layer having the lowest concentrationof B (boron), and as it recedes from the insulating layer, theconcentration of B increases.

In addition, the memory layer may be configured in such a manner that atleast in a state after the final manufacturing process is completed, theconcentration of B is low in the vicinity of the interface with theinsulating layer, and as it recedes from the interface, theconcentration of B continuously increases.

According to another embodiment, there is provided a memory deviceincluding a memory element that maintains information through themagnetization state of a magnetic material, and two kinds ofinterconnects that intersect each other, wherein the memory element hasthe configuration of the above-described memory element according to theembodiment, the memory element is disposed between the two kinds ofinterconnects, and a current flows to the memory element in thelamination direction through the two kinds of interconnects, and therebya spin-polarized electron is injected into the memory element.

According to the configuration of the memory element of the embodiment,a memory element that retains information through a magnetization stateof a magnetic material is provided, a magnetization-fixed layer isprovided over the memory layer through an intermediate layer, theintermediate layer is formed of an insulating material, an electron thatis spin-polarized is injected in a lamination direction and thereby themagnetization direction of the memory layer is changed and recording ofinformation is performed with respect to the memory layer, and thereforeit is possible to perform the recording of the information by flowing acurrent in the lamination direction and by injecting a spin-polarizedelectron.

In addition, the magnitude of an effective diamagnetic field which thememory layer receives is smaller than a saturated magnetization amountof the memory layer, such that it is possible to diminish the amount ofthe write current necessary for inverting the magnetization direction ofthe memory layer.

On the other hand, it is possible to diminish the amount of the writecurrent even when the amount of the saturated magnetization of thememory layer is not diminished, such that the amount of the saturatedmagnetization becomes sufficient, and it is possible to sufficientlysecure thermal stability of the memory layer.

Particularly, in the memory layer, the concentration of B is low in thevicinity of the interface with the insulating layer, and theconcentration of B is modulated such that the concentration of Bincreases, as it recedes from the insulating layer. That is, B diffusestoward the opposite side of the insulating layer. In this case, theopportunity for the Co or Fe of the memory layer to combine with oxygenwithin the insulating layer increases, such that interface magneticanisotropy becomes stronger. On the other hand, the concentration of Bincreases at a side away from the insulating layer, such that thesaturated magnetization decreases and therefore the diamagnetic fieldcomponent decreases.

In this manner, perpendicular magnetic anisotropy is enhanced.

In addition, according to the configuration of the memory device of theembodiment, a memory element having a memory layer that retainsinformation through a magnetization state of a magnetic material, andtwo kinds of interconnects that intersect each other are provided, thememory element has the configuration of the above-described memoryelement according to the embodiment, the memory element is disposedbetween the two kinds of interconnects, and a current flows to thememory element in the lamination direction through the two kinds ofinterconnects, and thereby a spin-polarized electron is injected to thememory element. Therefore, it is possible to perform the recording ofinformation by a spin injection by flowing a current in the laminationdirection of the memory element through the two interconnects.

In addition, even when the amount of the saturated magnetization is notdiminished, it is possible to diminish the amount of the write currentof the memory element, such that it is possible to stably retain theinformation recorded in the memory element and it is possible todiminish the power consumption of the memory device.

According to the embodiments, the amount of the write current isdiminished, and thermal stability representing the information retainingability is sufficiently secured, such that it is possible to configure amemory element excellent in a characteristic balance. Particularly, theconcentration of B at the interface with the insulating layer that is anoxide film is decreased, such that perpendicular magnetic anisotropy isenhanced. In addition, the concentration of B increases as it recedesfrom the interface with the insulating layer, such that the saturatedmagnetization decreases, and thereby perpendicular magnetic anisotropyis further enhanced.

Therefore, it is possible to realize a memory device that stablyoperates with high reliability.

In addition, the write current is diminished, such that it is possibleto diminish power consumption during performing writing into the memoryelement.

Therefore, it is possible to diminish the power consumption of theentirety of the memory device.

Additional features and advantages are described herein, and will beapparent from the following Detailed Description and the figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is an explanatory view illustrating a schematic configuration ofa memory device according to an embodiment;

FIG. 2 is a cross-sectional view illustrating a memory element accordingthe embodiment;

FIG. 3 is a diagram illustrating a relationship between an amount of Coof a memory layer of 0.09×0.18 μm size and an inverted current density;

FIG. 4 is a diagram illustrating a relationship between an amount of Coof a memory layer of 0.09×0.18 μm size and an index of thermalstability;

FIG. 5 is a diagram illustrating a relationship between an amount of Coof a memory layer of 50 nm φ size and an index of thermal stability;

FIGS. 6A to 6C are explanatory views illustrating a specimen of a memorylayer structure of the embodiment in which a concentration of B ismodulated;

FIG. 7 is an explanatory view illustrating a continuous modulationexample of the concentration of B of the memory layer according to theembodiment;

FIG. 8 is a perspective view illustrating a configuration of an MRAM inthe related art;

FIG. 9 is an explanatory view illustrating a schematic configuration ofthe memory device using magnetization inversion by spin injection; and

FIG. 10 is a cross-sectional view illustrating the memory of FIG. 7.

DETAILED DESCRIPTION

Embodiments of the present application will be described below in detailwith reference to the drawings.

1. Outline of Memory Element of Embodiment

2. Configuration of Embodiment

3. Experiment

1. Outline of Memory Element of Embodiment

First, an outline of a memory element of an embodiment according to thepresent application will be described.

The embodiment according to the present application performs therecording of information by inverting a magnetization direction of amemory layer of a memory element by the above-described spin injection.

The memory layer is formed of a magnetic material such as aferromagnetic layer, and retains information through the magnetizationstate (magnetization direction) of the magnetic material.

It will be described later in detail, but the memory element has alayered structure whose example is shown in FIG. 2, and includes amemory layer 17 and a magnetization-fixed layer 15 as two ferromagneticlayers, and an insulating layer 16 (tunnel insulating layer) as anintermediate layer provided between the two magnetic layers.

The memory layer 17 has a magnetization perpendicular to a film face anda magnetization direction varies corresponding to information.

The magnetization-fixed layer 15 has a magnetization that is a referencefor the information stored in the memory layer 17 and is perpendicularto the film face.

The insulating layer 16 is formed of a non-magnetic material and isprovided between the memory layer 17 and the magnetization-fixed layer15. In this embodiment, the insulating layer 16 is formed of an oxidefilm, for example, MgO (oxide magnesium).

Spin-polarized electrons are injected in the lamination direction of alaminated structure having the memory layer 17, the insulating layer 16,and the magnetization-fixed layer 15, and the magnetization direction ofthe memory layer 17 is changed and thereby information is recorded inthe memory layer 17.

A basic operation for inverting the magnetization direction of themagnetic layer (memory layer 17) by spin injection is to make a currentof a threshold value or greater flow to the memory element including agiant magnetoresistive effect element (GMR element) or a tunnelmagnetoresistive effect element (MTJ element) in a directionperpendicular to a film face. At this time, the polarity (direction) ofthe current depends on the inverted magnetization direction.

In a case where a current having an absolute value less than thethreshold value is made to flow, magnetization inversion does not occur.

A threshold value Ic of a current, which is necessary when themagnetization direction of the magnetic layer is inverted by spininjection, is expressed by the following Equation (1).

Ic=A·α·Ms·V·Hd/2η  (1)

Here, A: constant, α: spin braking constant, η: spin injectionefficiency, Ms: saturated magnetization amount, V: volume of memorylayer, and Hd: effective diamagnetic field.

As expressed by the Equation (1), a threshold value of a current may beset to an arbitrary value by controlling the volume V of the magneticlayer, the saturated magnetization Ms of the magnetic layer, and thespin injection efficiency η, and the spin braking constant α,

In this embodiment, the memory element includes the magnetic layer(memory layer 17) that is capable of retaining information through themagnetization state, and the magnetization-fixed layer 15 whosemagnetization direction is fixed.

The memory element has to retain written information so as to functionas a memory. This is determined by a value of an index Δ (=KV/k_(B)T) ofthermal stability as an index of ability of retaining information. Theabove-described Δ is expressed by the following Equation (2).

Δ=KV/k _(B) T=Ms·V·H _(k)·(½k _(B) T)  (2)

Here, H_(k): effective anisotropy field, k_(B): Boltzmann's constant, T:temperature, Ms: saturated magnetization amount, and V: volume of memorylayer.

The effective anisotropy field H_(k) receives an effect by a shapemagnetic anisotropy, an induced magnetic anisotropy, and a crystalmagnetic anisotropy, or the like, and when assuming a coherent rotationmodel of a single domain, the effective anisotropy field becomes thesame as the coercive force.

The index Δ of the thermal stability and the threshold value Ic of thecurrent are often in a trade-off relationship. Therefore, compatibilityof these becomes an issue to retain the memory characteristic.

In regard to the threshold value of the current that changes themagnetization state of the memory layer 17, actually, for example, in aTMR element in which the thickness of the memory layer 17 is 2 nm, and aplanar pattern is substantially an elliptical shape of 100 nm×150 nm, athreshold value +Ic of a positive side is +0.5 mA, a threshold value −Icof a negative side is −0.3 mA, and a current density at this time issubstantially 3.5×10⁶ A/cm². These substantially correspond to theabove-described Equation (1).

On the contrary, in a common MRAM that performs a magnetizationinversion using a current magnetic field, a write current of several mAor more is necessary.

Therefore, in case of performing magnetization inversion by spininjection, the threshold value of the above-described write currentbecomes sufficiently small, such that this is effective for diminishingpower consumption of an integrated circuit.

In addition, an interconnect (interconnect 105 of FIG. 8) for thegeneration of the current magnetic field, which is necessary for acommon MRAM, is not necessary, such that in regard to the degree ofintegration, it is advantageous compared to the common MRAM.

In the case of performing magnetization inversion by spin injection,since the writing (recording) of information is performed by directlyflowing a current to the memory element, to select a memory cell thatperforms the writing, the memory element is connected to a selectiontransistor to construct the memory cell.

In this case, the current flowing to the memory element is restricted toa current magnitude (saturated current of the selection transistor) thatcan be made to flow to the selection transistor.

To make the threshold value Ic of a current of the magnetizationinversion by the spin injection smaller than the saturated current ofthe selection transistor, as can be seen from the Equation (1), it iseffective to diminish the saturated magnetization amount Ms of thememory layer 17.

However, in the case of simply diminishing the saturated magnetizationamount Ms (for example, U.S. Pat. No. 7,242,045), the thermal stabilityof the memory layer 17 is significantly deteriorated, and therefore itis difficult for the memory element to function as a memory.

To construct the memory, it is necessary that the index Δ of the thermalstability is equal to or greater than a magnitude of a certain degree.

The present inventors have made various studies, and as a resultthereof, they have found that when for example, a composition of Co—Fe—Bis selected as the ferromagnetic layer making up the memory layer 17,the magnitude of the effective diamagnetic field (Meffective) which thememory layer 17 receives becomes smaller than the saturatedmagnetization amount Ms of the memory layer 17.

By using the above-described ferromagnetic material, the magnitude ofthe effective diamagnetic field which the memory layer 17 receivesbecomes smaller than the saturated magnetization amount Ms of the memorylayer 17.

In this manner, it is possible to make the diamagnetic field which thememory layer 17 receives small, such that it is possible to obtain aneffect of diminishing the threshold value Ic of a current expressed bythe Equation (1) without deteriorating the thermal stability Δ expressedby the Equation (2).

In addition, the present inventors has found that Co—Fe—B magnetizes ina direction perpendicular to a film face within a restricted compositionrange of the selected Co—Fe—B composition, and due to this, it ispossible to secure a sufficient thermal stability even in the case of anextremely minute memory element capable of realizing Gbit classcapacity.

Therefore, in regard to a spin injection type memory of the Gbit class,in a state where the thermal stability is secured in the a spininjection type memory of the Gbit class, it is possible to make a stablememory in which information may be written with a low current.

In this embodiment, it is configured such that the magnitude of theeffective diamagnetic field which the memory layer 17 receives is madeto be less than the saturated magnetization amount Ms of the memorylayer 17, that is, a ratio of the magnitude of the effective diamagneticfield with respect to the saturated magnetization amount Ms of thememory layer 17 becomes less than 1.

In addition, a magnetic tunnel junction (MTJ) element is configured byusing a tunnel insulating layer (insulating layer 16) formed of aninsulating material as the non-magnetic intermediate layer disposedbetween the memory layer 17 and the magnetization-fixed layer 15 inconsideration of the saturated current value of the selectiontransistor.

The magnetic tunnel junction (MTJ) element is configured by using thetunnel insulating layer, such that it is possible to make amagnetoresistance change ratio (MR ratio) large compared to a case wherea giant magnetoresistive effect (GMR) element is configured by using anon-magnetic conductive layer, and therefore it is possible to increasethe read-out signal strength.

Particularly, when magnesium oxide (MgO) is used as the material of thetunnel insulating layer 16, it is possible to make the magnetoresistancechange ratio (MR ratio) large compared to a case where aluminum oxide,which can be generally used, is used.

In addition, generally, spin injection efficiency depends on the MRratio, and the larger the MR ratio, the more spin injection efficiencyis improved, and therefore it is possible to diminish the magnetizationinversion current density.

Therefore, when magnesium oxide is used as the material of the tunnelinsulating layer 16 and the memory layer 17 is used, it is possible todiminish the threshold write current by spin injection and therefore itis possible to perform the writing (recording) of information with asmall current. In addition, it is possible to increase the read-outsignal strength.

In this manner, it is possible to diminish the threshold write currentby spin injection by securing the MR ratio (TMR ratio), and it ispossible to perform the writing (recording) of information with a smallcurrent. In addition, it is possible to increase the read-out signalstrength.

As described above, in a case where the tunnel insulating layer 16 isformed of the magnesium oxide (MgO) film, it is desirable that the MgOfilm is crystallized and therefore a crystal orientation is maintainedin 001 direction.

In addition, in this embodiment, in addition to a configuration formedof the magnesium oxide, the insulating layer 16 disposed between thememory layer 17 and the magnetization-fixed layer 15 may be configuredby using, for example, various insulating materials, dielectricmaterials, and semiconductors such as aluminum oxide, SiO₂, Bi₂O₃,SrTiO₂, AlLaO₃, and Al—N—O.

An area resistance value of the tunnel insulating layer 16 is necessaryto be controlled to several tens Ωμm² or less in consideration of theviewpoint of obtaining a current density necessary for inverting themagnetization direction of the memory layer 17 by spin injection.

In the tunnel insulating layer 16 formed of the MgO film, to retain thearea resistance value within the above-described range, it is necessaryto set the film thickness of the MgO film to 1.5 nm or less.

In addition, it is desirable to make the memory element small to easilyinvert the magnetization direction of the memory layer 17 with a smallcurrent.

Therefore, preferably, the area of the memory element is set to 0.01 μm²or less.

Furthermore, in this embodiment, the composition of the Co—Fe—B magneticlayer making up the memory layer 17 is modulated in the directionperpendicular to the film face. Particularly, a concentration of B islow at an interface with the insulating layer 16 (oxide film of MgO),and as it recedes from the interface, the concentration of B increasescontinuously or in a step wise fashion.

The concentration of B at the interface with the insulating layer 16that is an oxide film is decreased, such that perpendicular magneticanisotropy is enhanced. This is because a Co—O combination or a Fe—Ocombination that is regarded as an origin of interface perpendicularmagnetic anisotropy is difficult for B to hinder. In addition, theconcentration of B increases as it recedes from the interface, such thatthe saturated magnetization decreases, and therefore perpendicularmagnetic anisotropy is further enhanced.

It is desirable that the magnetization-fixed layer 15 and the memorylayer 17 have a unidirectional anisotropy.

In addition, it is preferable that the film thickness of each of themagnetization-fixed layer 15 and the memory layer 17 be 0.5 to 30 nm.

Other configuration of the memory element may be the same as theconfiguration of a memory element that records information by spininjection in the related art.

The magnetization-fixed layer 15 may be configured in such a manner thatthe magnetization direction is fixed by only a ferromagnetic layer or byusing an anti-ferromagnetic combining of an anti-ferromagnetic layer anda ferromagnetic layer.

In addition, the magnetization-fixed layer 15 may be configured by asingle layer of a ferromagnetic layer, or a ferri-pin structure in whicha plurality of ferromagnetic layers are laminated through a non-magneticlayer.

As a material of the ferromagnetic layer making up themagnetization-fixed layer 15 of the laminated ferri-pin structure, Co,CoFe, CoFeB, or the like may be used. In addition, as a material of thenon-magnetic layer, Ru, Re, Ir, Os, or the like may be used.

As a material of the anti-ferromagnetic layer, a magnetic material suchas an FeMn alloy, a PtMn alloy, a PtCrMn alloy, an NiMn alloy, an IrMnalloy, NiO, and Fe₂O₃ may be exemplified.

In addition, a magnetic characteristic may be adjusted by adding anon-magnetic element such as Ag, Cu, Au, Al, Si, Bi, Ta, B, C, O, N, Pd,Pt, Zr, Hf, Ir, W, Mo, and Nb to the above-describe magnetic materials,or in addition to this, various physical properties such as acrystalline structure, a crystalline property, a stability of asubstance, or the like may be adjusted.

In addition, in relation to a film configuration of the memory element,the memory layer 17 may be disposed at the lower side of themagnetization-fixed layer 15, or at the upper side thereof, and in anydisposition, there is no problem at all. In addition, there is noproblem at all in a case where the magnetization-fixed layer 15 isdisposed at the upper side and the lower side of the memory layer 17,so-called dual-structure.

In addition, as a method of reading-out information recorded in thememory layer 17 of the memory element, a magnetic layer that becomes areference for the information is provided on the memory layer 17 of thememory element through a thin insulating film, and the reading-out maybe performed by a ferromagnetic tunnel current flowing through theinsulating layer 16, or the reading-out may be performed by amagnetoresistive effect.

2. Configuration of Embodiment

Subsequently, a specific configuration of this embodiment will bedescribed.

As an embodiment, a schematic configuration diagram (perspective view)of a memory device is shown in FIG. 1.

This memory device includes a memory element 3, which can retaininformation at a magnetization state, disposed in the vicinity of anintersection of two kinds of address interconnects (for example, a wordline and a bit line) that are perpendicular to each other.

Specifically, a drain region 8, a source region 7, and a gate electrode1 that make up a selection transistor that selects each memory cell areformed in a portion separated by an element separation layer 2 of asemiconductor substrate 10 such as a silicon substrate, respectively.Among them, the gate electrode 1 also functions as one side addressinterconnect (for example, a word line) that extends in the front-backdirection in the drawing.

The drain region 8 is formed commonly with left and right selectiontransistors in the drawing, and an interconnect 9 is connected to thedrain region 8.

The memory element 3 is disposed between the source region 7, and theother side address interconnect (for example, a bit line) 6 that isdisposed at the upper side and extends in the left-right direction inthe drawing. This memory element 3 has a memory layer including aferromagentic layer whose magnetization direction is inverted by spininjection.

In addition, the memory element 3 is disposed in the vicinity of anintersection of two kinds of address interconnects 1 and 6.

The memory element 3 is connected to the bit line 6 and the sourceregion 7 through upper and lower contact layers 4, respectively.

In this manner, a current flows into the memory element 3 in theperpendicular direction thereof through the two kind of addressinterconnects 1 and 6, and the magnetization direction of the memorylayer may be inverted by spin injection.

In addition, a cross-sectional view of the memory element 3 of thememory device according to this embodiment is shown in FIG. 2.

As shown in FIG. 2, in the memory element 3, an underlying layer 14, themagnetization-fixed layer 15, the insulating layer 16, the memory layer17, and the cap layer 18 are laminated in this order from a lower layerside.

In this case, the magnetization-fixed layer 15 is provided at a lowerlayer in relation to a memory layer 17 in which the magnetizationdirection of a magnetization M17 is inverted by spin injection.

In regard to the spin injection type memory, “0” and “1” of informationare defined by a relative angle between the magnetization M17 of thememory layer 17 and a magnetization M15 of the magnetization-fixed layer15.

An intermediate layer 16 that serves as a tunnel barrier layer (tunnelinsulating layer) is provided between the memory layer 17 and themagnetization-fixed layer 15, and therefore an MTJ element is configuredby the memory layer 17 and the magnetization-fixed layer 15.

In addition, an underlying layer 14 is formed under themagnetization-fixed layer 15, and a cap layer 18 is formed on the memorylayer 17.

The memory layer 17 is formed of a ferromagnetic material havingmagnetic moment in which the direction of magnetization M17 is freelychanged in a direction perpendicular to a film face. Themagnetization-fixed layer 15 is formed of a ferromagnetic materialhaving magnetic moment in which magnetization M15 is fixed in thedirection perpendicular to the film face.

The storage of information is performed by a magnetization direction ofthe memory layer 17 having unidirectional anisotropy. The writing ofinformation is performed by applying a current in the directionperpendicular to the film face and by causing a spin torquemagnetization inversion. In this way, the magnetization-fixed layer 15is provided at a lower layer in relation to the memory layer 17 in whichthe magnetization direction is inverted by the spin injection, andserves as a reference for the memory information (magnetizationdirection) of the memory layer 17.

In this embodiment, Co—Fe—B is used for the memory layer 17 and themagnetization-fixed layer 15.

The magnetization-fixed layer 15 serves as the reference for theinformation, such that it is necessary that the magnetization directiondoes not vary, but it is not necessarily necessary to be fixed in aspecific direction. The magnetization-fixed layer 15 may be configuredin such a manner that migration becomes more difficult than in thememory layer 17 by making a coercive force large, by making the filmthickness large, or by making a damping constant large compared to thememory layer 17.

In the case of fixing the magnetization, an anti-ferromagnetic materialsuch as PtMn and IrMn may be brought into contact with themagnetization-fixed layer 15, or a magnetic material brought intocontact with such an anti-ferromagnetic material may be magneticallycombined through a non-magnetic material such as Ru, and thereby themagnetization-fixed layer 15 may be indirectly fixed.

In this embodiment, particularly, a composition of the memory layer 17of the memory element 3 is adjusted such that a magnitude of aneffective diamagnetic field which the memory layer 17 receives becomessmaller than the saturated magnetization amount Ms of the memory layer17.

That is, as described above, a composition of a ferromagnetic materialCo—Fe—B of the memory layer 17 is selected, and the magnitude of theeffective diamagnetic field which the memory layer 17 receives is madeto be small, such that the magnitude of the effective diamagnetic fieldbecomes smaller than the saturated magnetization amount Ms of the memorylayer 17.

In addition, in this embodiment, in a case where the insulating layer 16that is an intermediate layer is formed of a magnesium oxide layer. Inthis case, it is possible to make a magnetoresistive change ratio (MRratio) high.

When the MR ratio is made to be high as described above, the spininjection efficiency is improved, and therefore it is possible todiminish a current density necessary for inverting the direction of themagnetization M17 of the memory layer 17.

In addition, as described above, the composition of the Co—Fe—B magneticlayer making up the memory layer 17 is modulated in the directionperpendicular to the film face, and the composition is designed in sucha manner that a concentration of B is low at an interface with theinsulating layer 16 (oxide film of MgO), and as it recedes from theinterface (that is, toward the cap layer 18), the concentration of Bincreases continuously or in a step wise fashion.

The memory element 3 of this embodiment can be manufactured bycontinuously forming from the underlying layer 14 to the cap layer 18 ina vacuum apparatus, and then by forming a pattern of the memory element3 by a processing such as a subsequent etching or the like.

According to the above-described embodiment, the memory layer 17 of thememory element 3 is configured in such a manner that the magnitude ofthe effective diamagnetic field that the memory layer 17 receives issmaller than the saturated magnetization amount Ms of the memory layer17, such that the diamagnetic field that the memory layer 17 receives isdecreased, and it is possible to diminish an amount of the write currentnecessary for inverting the direction of the magnetization M17 of thememory layer 17.

On the other hand, since the amount of the write current may bediminished even when the saturated magnetization amount Ms of the memorylayer 17 is not diminished, it is possible to sufficiently secure theamount of the saturated magnetization of the memory layer 17 andtherefore it is possible to sufficiently secure the thermal stability ofthe memory layer 17.

As described above, since it is possible to sufficiently secure thethermal stability that is an information retaining ability, it ispossible to configure the memory element 3 excellent in a characteristicbalance.

In this manner, an operation error is removed and an operation margin ofthe memory element 3 is sufficiently obtained, such that it is possibleto stably operate the memory element 3.

Accordingly, it is possible to realize a memory device that operatesstably with high reliability.

In addition, the write current is diminished, such that it is possibleto diminish the power consumption when performing the writing into thememory element 3.

Therefore, it is possible to diminish the power consumption of theentirety of the memory in which a memory cell is configured by thememory element 3 of this embodiment.

Therefore, in regard to the memory device including the memory element 3capable of realizing a memory that is excellent in information retainingability, has high reliability, and operates stably, it is possible todiminish the power consumption in a memory device including the memoryelement 3.

In addition, the memory that includes a memory element 3 shown in FIG. 2and has a configuration shown in FIG. 1 has an advantage in that ageneral semiconductor MOS forming process may be applied when the memoryis manufactured.

Therefore, it is possible to apply the memory device of this embodimentas a general purpose memory.

3. Experiment

Here, in regard to the configuration of the memory element of thisembodiment, by specifically selecting the material of the ferromagneticlayer making up the memory layer 17, the magnitude of the effectivediamagnetic field that the memory layer 17 receives was adjusted, andthereby a sample of the memory element 3 was manufactured, and thencharacteristics thereof was examined.

In an actual memory device, as shown in FIG. 1, a semiconductor circuitfor switching or the like present in addition to the memory element 3,but here, the examination was made on a wafer in which only the memoryelement is formed for the purpose of investigating a magnetizationinversion characteristic of the memory layer 17.

Experiment 1

A thermal oxide film having a thickness of 300 nm was formed on asilicon substrate having a thickness of 0.725 mm, and the memory element3 having a configuration shown in FIG. 2 was formed on the thermal oxidefilm.

Specifically, in regard to the memory element 3 shown in FIG. 2, amaterial and a film thickness of each layer were selected as describedbelow.

-   -   Underlying layer 14: Laminated film of a Ta film having a film        thickness of 10 nm and a Ru film having a film thickness of 25        nm    -   Magnetization-fixed layer 15: CoFeB film having a film thickness        of 2.5 nm    -   Tunnel insulating layer 16: Magnesium oxide film having a film        thickness of 0.9 nm    -   Memory layer 17: CoFeB film having the same composition as that        of the magnetization-fixed layer    -   Cap layer 18: Laminated film of a Ta film having a film        thickness of 3 nm, a Ru film having a thickness of 3 nm, and a        Ta film having a thickness of 3 nm

Each layer was selected as described above, a Cu film (not shown) havinga film thickness of 100 nm (serving as a word line described below) wasprovided between the underlying layer 14 and the silicon substrate.

In the above-described configuration, the ferromagnetic layer of thememory layer 17 was formed of a ternary alloy of Co—Fe—B, and a filmthickness of the ferromagnetic layer was fixed to 2.0 nm.

Each layer other than the insulating layer 16 formed of a magnesiumoxide film was formed using a DC magnetron sputtering method.

The insulating layer 16 formed of the magnesium oxide (MgO) film wasformed using a RF magnetron sputtering method.

In addition, after forming each layer of the memory element 3, a heatingtreatment was performed in a magnetic field heat treatment furnace.

Next, after masking a word line portion by a photolithography, aselective etching by Ar plasma was performed with respect to a laminatedfilm other than the word line portion, and thereby the word line (lowerelectrode) was formed.

At this time, a portion other than the word line was etched to the depthof 5 nm in the substrate.

Then, a mask of a pattern of the memory element 3 by an electron beamdrawing apparatus was formed, a selective etching was performed withrespect to the laminated film, and thereby the memory element 3 wasformed. A portion other than the memory element 3 was etched to aportion of the word line immediately over the Cu layer.

In addition, in the memory element for the characteristic evaluation, itis necessary to make a sufficient current flow to the memory element soas to generate a spin torque necessary for the magnetization inversion,such that it is necessary to suppress the resistance value of the tunnelinsulating layer. Therefore, a pattern of the memory element 3 was setto an elliptical shape having a short axis of 0.09 μm×a long axis of0.18 μm, and an area resistance value (Ωμm2) of the memory element 3 wasset to 20 Ωμm2.

Next, a portion other than the memory element 3 was insulated bysputtering Al2O3 to have a thickness of substantially 100 nm.

Then, a bit line serving as an upper electrode and a measurement padwere formed by using photolithography.

In this manner, a sample of the memory element 3 was manufactured.

By the above-described manufacturing method, each sample of the memoryelement 3 in which a composition of Co—Fe—B alloy of the ferromagneticlayer of the memory layer 17 was changed was manufactured.

In the composition of the Co—Fe—B alloy, a composition ratio of CoFe andB was fixed to 80:20, and a composition ratio of Co in CoFe, that is,x(atomic %) was changed to 90%, 80%, 70%, 60%, 50%, 40%, 30%, 20%, 10%,and 0%.

With respect to each sample of the memory element 3 manufactured asdescribed above, a characteristic evaluation was performed as describedbelow.

Before the measurement, it was configured to apply a magnetic field tothe memory element 3 from the outside to control an inversion current insuch a manner that a value in a plus direction and a value in a minusdirection to be symmetric to each other.

In addition, a voltage applied to the memory element 3 was set up to 1 Vwithin a range without breaking down the insulating 16.

Measurement of Saturated Magnetization Amount

The saturated magnetization amount Ms was measured by a VSM measurementusing a Vibrating Sample Magnetometer.

Measurement of Effective Diamagnetic Field

As a sample for measuring an effective diamagnetic field, in addition tothe above-described sample of the memory element 3, a sample in whicheach layer making up the memory element 3 was formed was manufacturedand then the sample was processed to have a planar pattern of 20 mm×20mm square.

In addition, a magnitude M_(effective) of an effective diamagnetic fieldwas obtained by FMR (Ferromagnetic Resonance) measurement.

A resonance frequency fFMR, which is obtained by the FMR measurement,with respect to arbitrary external magnetic field H_(ex) is given by thefollowing Equation (3).

Math. 1

|f _(FMR)=γ′√{square root over (4πM _(effective)(H _(K) +H _(ex)))}  (3)

Here, M_(effective) in the Equation (3) may be expressed by 4πM_(effective)=4π Ms−H⊥ (H⊥: anisotropy field in a directionperpendicular to a film face).

Measurement of Inversion Current Value and Thermal Stability

An inversion current value was measured for the purpose of evaluatingthe writing characteristic of the memory element 3 according to thisembodiment.

A current having a pulse width of 10 μs to 100 ms is made to flow to thememory element 3, and then a resistance value of the memory element 3was measured.

In addition, the amount of current that flows to the memory element 3was changed, and then a current value at which a direction of themagnetization M17 of the memory layer 17 of the memory element 3 wasinverted was obtained. A value obtained by extrapolating a pulse widthdependency of this current value to a pulse width 1 ns was set to theinversion current value.

In addition, the inclination of a pulse width dependency of theinversion current value corresponds to the above-described index Δ ofthe thermal stability of the memory element 3. The less the inversioncurrent value is changed (the inclination is small) by the pulse width,the more the memory element 3 is strong against disturbance of the heat.

In addition, twenty memory elements 3 with the same configuration weremanufactured to take variation in the memory element 3 itself intoconsideration, the above-described measurement was performed, and anaverage value of the inversion current value and the index Δ of thethermal stability were obtained.

In addition, an inversion current density Jc0 was calculated from theaverage value of the inversion current value obtained by the measurementand an area of the planar pattern of the memory element 3.

In regard to each sample of the memory element 3, a composition ofCo—Fe—B alloy of the memory layer 17, measurement results of thesaturated magnetization amount Ms and the magnitude M_(effective) of theeffective diamagnetic field, and a ratio M_(effective)/Ms of effectivediamagnetic field to the saturated magnetization amount were shown inTable 1. Here, an amount of Co of Co—Fe—B alloy of the memory layer 17described in Table 1 was expressed by an atomic %.

TABLE 1 Ms (emu/cc) Meffctive (emu/cc) Meffective/Ms (Co₉₀Fe₁₀)₈₀—B₂₀960 1210 1.26 (Co₈₀Fe₂₀)₈₀—B₂₀ 960 1010 1.05 (Co₇₀Fe₃₀)₈₀—B₂₀ 1040 9000.87 (Co₆₀Fe₄₀)₈₀—B₂₀ 1200 830 0.69 (Co₅₀Fe₅₀)₈₀—B₂₀ 1300 690 0.53(Co₄₀Fe₆₀)₈₀—B₂₀ 1300 500 0.38 (Co₃₀Fe₇₀)₈₀—B₂₀ 1260 390 0.31(Co₂₀Fe₈₀)₈₀—B₂₀ 1230 360 0.29 (Co₁₀Fe₉₀)₈₀—B₂₀ 1200 345 0.29 Fe₈₀—B₂₀1160 325 0.28

From the table 1, in a case where the amount x of Co in(Co_(x)Fe_(100-x))₈₀B₂₀ was 70% or less, the magnitude of the effectivediamagnetic field (M_(effective)) was smaller than the saturatedmagnetization amount Ms, that is, the ratio of M_(effective)/Ms in acase where the amount x of Co was 70% or less become a value less than1.0.

In addition, it was confirmed that the more the amount x of Codecreased, the larger the difference between M_(effective) and Ms.

A measurement result of the inversion current value was shown in FIG. 3,and a measurement result of the index of the thermal stability was shownin FIG. 4.

FIG. 3 shows a relationship between the amount x (content in CoFe;atomic %) of Co in the Co—Fe—B alloy of the memory layer 17 and theinversion current density Jc0 obtained from the inversion current value.

FIG. 4 shows a relationship between the amount x (content in CoFe;atomic %) of Co of the Co—Fe—B alloy of the memory layer 17 and theindex Δ(KV/k_(B)T) of the thermal stability.

As can be seen from FIG. 3, as the amount x of Co decreases, theinversion current density Jc0 decreases.

This is because in a case where the amount x of Co becomes small, thesaturated magnetization amount Ms increases, but the effectivediamagnetic field M_(effective) decreases, and therefore the product ofthem Ms×M_(effective) becomes small.

As can be seen from FIG. 4, as the amount x of Co decreased, the index Δ(KV/k_(B)T) of the thermal stability increased, and in a case where theamount x of Co became more or less small to a degree, the index Δ of thethermal stability became stable to a large value.

This well corresponds to a change that is expected from the measurementresult of the saturated magnetization amount Ms shown in Table 1 and atendency where the index Δ of the thermal stability from the Equation(2) is proportional to the saturated magnetization amount Ms.

As was clear from the results of Table 1, FIGS. 3 and 4, in acomposition where the amount x of Co was 70% or less and the effectivediamagnetic field M_(effective) was less than the saturatedmagnetization amount Ms, it was possible to diminish the inversioncurrent value Jc0 with a high thermal stability maintained, withoutusing a method in which Ms was decreased and therefore the thermalstability was sacrificed.

Experiment 2

As could be seen from the Experiment 1, in the case of(Co_(x)Fe_(100-x))₈₀B₂₀, it was possible to diminish the inversioncurrent value Jc0 with a high thermal stability maintained in acomposition where the amount x of Co was 70% or less.

Therefore, in Experiment 2, an effect on a ratio of Co and Fe, and theM_(effective)/MS, which was caused by an amount of B, was examined byusing a memory layer 17 having a composition (CO₇₀Fe₃₀)₈₀B_(z) and acomposition (CO₈₀Fe₂₀)₈₀B_(z). The details of a sample weresubstantially the same as those in the Experiment 1.

Table 2 shows compositions of CoFeB in which the amount of B was set to5 to 40% in (CO₇₀Fe₃₀)_(100-z)B_(z), results of measurement of thesaturated magnetization amount Ms and the magnitude M_(effective) of theeffective diamagnetic field, and a ratio M_(effective)/MS of thesaturated magnetization amount and the magnitude of the effectivediamagnetic field.

In addition, Table 3 shows compositions of CoFeB in which the amount ofB was similarly set to 5 to 40% in (CO₈₀Fe₂₀)_(100-z)B_(z), and a ratioM_(effective)/MS of the saturated magnetization amount and the magnitudeof the effective diamagnetic field.

TABLE 2 Ms (emu/cc) Meffective (emu/cc) Meffective/Ms (Co₇₀Fe₃₀)₉₅—B₅)1310 1090 0.83 (Co₇₀Fe₃₀)₉₀—B₁₀) 1250 1080 0.89 (Co₇₀Fe₃₀)₈₀—B₂₀) 1040900 0.87 (Co₇₀Fe₃₀)₇₀—B₃₀) 820 730 0.89 (Co₇₀Fe₃₀)₆₀—B₄₀) 450 690 1.53

TABLE 3 Ms (emu/cc) Meffective (emu/cc) Meffective/Ms (Co₈₀Fe₂₀)₉₅—B₅)1250 1280 1.02 (Co₈₀Fe₂₀)₉₀—B₁₀) 1100 1140 1.04 (Co₈₀Fe₂₀)₈₀—B₂₀) 9601010 1.05 (Co₈₀Fe₂₀)₇₀—B₃₀) 750 890 1.19 (Co₈₀Fe₂₀)₆₀—B₄₀) 430 690 1.60

From the results of Table 2, it can be confirmed that in a case wherethe ratio of Co and Fe was set to 70/30 like (CO₇₀Fe₃₀)_(100-z)B_(z),the magnitude M_(effective) of the effective diamagnetic field wassmaller than the saturated magnetization amount Ms in compositions otherthan a composition where the amount z of B was 40 atomic %.

From the results of Table 3, it can be confirmed that in a case wherethe ratio of Co and Fe was set to 80/20 like (CO₈₀Fe₂₀)_(100-z)B_(z),the magnitude M_(effective) of the effective diamagnetic field waslarger than the saturated magnetization amount Ms in all compositions.

From the results of the above-described Tables 1 to 3, it was revealedthat in a case where the amount of B is within a range of 30 atomic % orless, a magnitude correlation of the amount of the saturatedmagnetization Ms and the magnitude M_(effective) of the effectivediamagnetic field is determined by the ratio of Co and Fe.

Therefore, a composition of the Co—Fe—B alloy where the magnitudeM_(effective) of the effective diamagnetic field of the memory layer 17is less than the amount of the saturated magnetization Ms is as follows:

(CO_(x)—Fe_(y))_(100-z)—B_(z),

Here, 0≦Co_(x)≦70,

30≦Fe_(y)≦100,

0<B_(z)≦30.

Experiment 3

In a spin injection type memory of Gbit class, it was assumed that thesize of the memory element is 100 nmφ. Therefore, in Experiment 3, thethermal stability was evaluated by using a memory element having thesize of 50 nmφ.

In the composition of Co—Fe—B alloy, a composition ratio (atomic %) ofCoFe and B was fixed, and a composition ratio x (atomic %) of Co in CoFewas changed to 90%, 80%, 70%, 60%, 50%, 40%, 30%, 20%, 10%, and 0%.

The details of the sample other than the sample size were substantiallythe same as those in the Experiment 1.

In a case where the size of the memory element 3 was 50 nmφ, arelationship between an amount of Co (content in CoFe; atomic %) in theCo—Fe—B alloy, and the index Δ (KV/k_(B)T) of the thermal stability wereshown in FIG. 5.

As can be seen from FIG. 5, when the element size was 50 nmφ, Co—Fe—Balloy composition dependency of the thermal stability index Δ waslargely varied from the Co—Fe—B alloy composition dependency of Δobtained in the elliptical memory element having a short axis of 0.09μm×a long axis of 0.18 μm shown in FIG. 4.

According to FIG. 5, the high thermal stability was maintained only inthe case of Co—Fe—B alloy composition where Fe is 60 atomic % or more.

As a result of various investigations, it was clear that the reason whythe Co—Fe—B alloy containing Fe of 60 atomic % or more shows the highthermal stability Δ in the extremely minute memory element was revealedto be because the magnetization of the Co—Fe—B alloy faced a directionperpendicular to a film face.

The reason why the magnetization of the Co—Fe—B alloy faces thedirection perpendicular to the film face is considered to be because ofa composition in which the effective diamagnetic field M_(effective) issignificantly smaller than the saturated magnetization amount Ms.

In addition, the reason why the thermal stability is secured even in thecase of the extremely minute element of a perpendicular magnetizationtype is related to Hk (effective anisotropy field) in the Equation (2),and Hk of the perpendicular magnetization film becomes a valuesignificantly larger than that in the in-plane magnetization film. Thatis, in the perpendicular magnetization film, due to an effect of largeHk, it is possible to maintain a high thermal stability Δ even in thecase of the extremely minute element not capable of securing asufficient thermal stability Δ in the in-plane magnetization film.

From the above-described experiment results, in regard to the Co—Fe—Balloy having a composition of (Co_(x)Fe_(100-x))₈₀B₂₀, in a case wherethe amount of Fe is 60% or more, this alloy may be said to be suitablefor the memory device of the Gbit class using spin injection.

Experiment 4

As could be seen from the above-described Experiment 3, in a case theamount of F was 60 or more in the Co—Fe—B alloy having a composition of(Co_(x)Fe_(100-x))₈₀B₂₀, this alloy was suitable for the memory deviceof the Gbit class using spin injection. In Experiment 4, a memoryelement having the size of 50 nmφ was manufactured using the Co—Fe—Balloy containing B in an amount of 5 to 30 atomic %, and the thermalstability was evaluated.

The details other than the element size were substantially the same asthose in the Experiment 1.

A relationship between the index Δ (KV/k_(B)T) of the thermal stabilityand the Co—Fe—B alloy having a composition(Co_(x)Fe_(100-x))_(100-z)B_(z) in which an amount x of Co was 50, 40,30, 20, 10, and 0, and an amount z of B was 5, 10, 20, and 30 was shownin Table 4.

TABLE 4 (Co₅₀—Fe₅₀)_(100−z)—B_(z)) (Co₄₀—Fe₆₀)_(100−z)—B_(z))(Co₃₀—Fe₇₀)_(100−z)—B_(z)) B_(z) = 5 atomic % 19 40 42 B_(z) = 10 atomic% 20 41.5 43 B_(z) = 20 atomic % 20 43 44 B_(z) = 30 atomic % 21 45 47(Co₂₀—Fe₈₀)_(100−z)—B_(z)) (Co₁₀—Fe₉₀)_(100−z)—B_(z)) Fe_(100−z)—B_(z)B_(z) = 5 atomic % 42 43 44 B_(z) = 10 atomic % 44 44 45 B_(z) = 20atomic % 45 46 46 B_(z) = 30 atomic % 48 48 48

As can be seen from Table 4, the thermal stability Δ in all compositionsexcept that a case where the amount x of Co was 50, and the amount z ofB was 5 to 30 was maintained to be large.

That is, as was the case with the result of the Experiment 4, it wasrevealed that the amount x of Co of 50 and 60 became a boundary line forsecuring high thermal stability in a extremely minute elementcorresponding to the spin injection type memory of the Gbit class.

Therefore, from the above-described result, it was revealed that theCo—Fe—B alloy of the memory layer 17 was suitable for manufacturing thespin injection type memory of the Gbit class in the followingcomposition:

(CO_(x)—Fe_(y))_(100-z)—B_(z),

Here, 0≦Co_(x)≦40,

60≦Fe_(y)≦100,

0<B_(z)≦30.

In addition, in regard to the Co—Fe—B alloy, in a composition where theratio of Fe was great in Co and Fe, the difference between theM_(effective) and Ms becomes large, and this alloy is apt to bemagnetized, and therefore it is easy to secure thermal stability.

Therefore, in a case where the capacity of the magnetic memory increasesand the size of the memory element 3 decreases, it is easy to securethermal stability in the Co—Fe—B alloy containing a large amount of Fe.

Therefore, for example, when a situation is assumed in which the spininjection type magnetic memory of the Gbit class is realized by thememory layer 17 in which the amount y of Fe is 60, and the size thereofis 70 nmφ, it is preferable that whenever the diameter of the memoryelement 3 decreases by 5 nmφ, the amount y of Fe in the Co—Fe—B alloyincreases by a value of 5.

For example, in the case of the (CO_(x)—Fe_(y))_(100-z)—B_(z), theamount y of Fe is set in a manner that an atomic % as a content in CoFeis 65%, 70%, 75%, 80%, . . . (in terms of the amount x of Co, 35%, 30%,25%, 20%, . . . ), and this is a more appropriate example to correspondto the size reduction of the memory element.

Experiment 5

Next, an experiment on a concentration modulation of B was performedwith respect to the memory layer 17 of the Co—Fe—B alloy.

FIG. 6A shows a configuration of the sample.

As was the case with the Experiments 1 to 4, in this case, a thermaloxide film having the thickness of 300 nm was formed on a siliconsubstrate having the thickness of 0.725 nm, and the memory element 3shown in FIG. 2 was formed on this oxide film.

A layer structure of the memory element 3 was set as shown in FIG. 6A.

-   -   Underlying layer 14: Laminated film of a Ta film having a film        thickness of 10 nm and a Ru film having a film thickness of 25        nm    -   Magnetization-fixed layer 15: CoFeB film having a film thickness        of 2.5 nm    -   Tunnel insulating layer 16: Magnesium oxide film having a film        thickness of 0.9 nm    -   Memory layer 17: CoFeB film having a total film thickness of 2.0        nm    -   Cap layer 18: Laminated film of a Ru film having a film        thickness of 3 nm, and a Ta film having the thickness of 3 nm

In addition, a Cu film (not shown) having a film thickness of 100 nm(serving as a word line described below) was provided between theunderlying layer 14 and the silicon substrate, and thereby each layerwas formed.

The composition of the Co—Fe—B alloy of the magnetization-fixed layer 15was set to (Co₃₀—Fe₇₀)₈₀—B₂₀. That is, in CoFe, Co and Fe were set to30% and 70%, respectively, the CoFe having this composition was set to80%, and B was set to 20%, by atomic %.

As shown in FIG. 6B, the memory layer 17 was configured in such a mannerthat four layers of Co—Fe—B film are laminated, each having a differentcomposition.

Each of this composition was, from a film that comes into contact withthe insulating layer 16, by atomic %,

(Co30%-Fe70%)95%-B5%,

(Co30%-Fe70%)85%-B15%,

(Co30%-Fe70%)80%-B20%, and

(Co30%-Fe70%)60%-B40%.

That is, the concentration of B was adjusted to be increased as itrecedes from the insulating layer 16 while the composition ratio ofCo—Fe was maintained constantly.

The film thickness was set in such a manner that all of them had 0.5 nm,and therefore the total thickness was 2.0 nm.

The sample having the above-described layer configuration was calledsample 3A.

As was the case with the Experiments 1 to 4, the sample 3A of the memoryelement 3 was manufactured as described below.

That is, Each layer other than the insulating layer 16 formed of amagnesium oxide film was formed using a DC magnetron sputtering method,and the insulating layer was formed using an RF magnetron sputteringmethod.

In addition, after forming each layer of the memory element 3, a heatingtreatment was performed in a magnetic field heat treatment furnace.

In addition, after masking a word line portion by photolithography,selective etching by Ar plasma was performed with respect to a laminatedfilm other than the word line portion, and thereby the word line (lowerelectrode) was formed. At this time, a portion other than the word linewas etched to the depth of 5 nm in the substrate.

Then, a mask of the pattern of the memory element 3 was formed by anelectron beam drawing apparatus, selective etching was performed withrespect to the laminated film, and thereby the memory element 3 wasformed. A portion other than the memory element 3 was etched to aportion of the word line immediately over the Cu layer.

In addition, in the memory element for characteristic evaluation, it isnecessary to make a sufficient current flow to the memory element so asto generate the spin torque necessary for magnetization inversion, suchthat it is necessary to suppress the resistance value of the tunnelinsulating layer. Therefore, the pattern of the memory element 3 was setto an elliptical shape having a short axis of 0.09 μm×a long axis of0.18 μm, and an area resistance value (Ωμm²) of the memory element 3 wasset to 20 Ωm².

Next, a portion other than the memory element 3 was insulated bysputtering Al₂O₃ to have a thickness of substantially 100 nm. Then, abit line serving as an upper electrode and a measurement pad were formedby using photolithography.

In addition, as shown in FIG. 6C, as a comparative example, the memorylayer 17 was formed by a single Co—Fe—B layer having a composition of(Co30%-Fe70%)80%-B20% (by atomic %), and the thickness of 2.0 nm, andthereby a sample 3B was manufactured.

With respect to each of the sample 3A and 3B of the memory element 3manufactured as described above, characteristic evaluation was performedas described below.

In addition, in this case, before the measurement, it was configured toapply a magnetic field to the memory element 3 from the outside tocontrol an inversion current in such a manner that a value in a plusdirection and a value in a minus direction to be symmetric to eachother. In addition, a voltage applied to the memory element 3 was set upto 1 V within a range without breaking down the insulating layer 16.

Measurement of Magnetization Curve

A magnetization curve of the memory element 3 of each sample 3A and 3Bwas measured by a VSM measurement using Vibrating Sample Magnetometer.

At this time, a bulk film portion having a size substantially of 8 mm×8mm prepared specially on a wafer for magnetization evaluation was usedinstead of an element subjected to micro machining. In addition, ameasurement magnetic field was applied in a direction perpendicular to afilm face.

Measurement of Inversion Current Value and Thermal Stability

A measurement of an inversion current value was performed for thepurpose of evaluating a writing characteristic.

A current of 10 μs to 100 ms was made to flow to the samples 3A and 3B,then a resistance value of the sample 3A and 3B. In addition, the amountof current made to flow to the samples 3A and 3B was changed, and then acurrent value at which the direction of the magnetization M17 of thememory layer 17 of the samples 3A and 3B was obtained. A value obtainedby extrapolating a pulse width dependency of this current value to apulse width 1 ns was set to the inversion current value.

In addition, an inclination of a pulse width dependency of the inversioncurrent value corresponds to the above-described index Δ of the thermalstability of the samples 3A and 3B. The less the inversion current valueis changed (the inclination is small) by the pulse width, the more thememory element 3 is strengthened against heat disturbance.

In addition, twenty samples 3A and 3B with the same configuration weremanufactured to take variations between the samples 3A and 3B intoconsideration, the above-described measurement was performed, and anaverage value of the inversion current value and the index Δ of thethermal stability were obtained.

In addition, an inversion current density Jc0 was calculated from theaverage value of the inversion current value obtained by the measurementand an area of the planar pattern of the samples 3A and 3B.

Coercive force of the samples 3A and 3B, which was obtained by the VSM,and values of Jc0 and Δ obtained from the measurement of the spininjection magnetization inversion were shown in Table 5.

TABLE 5 Hc [Oe] Jc0 [MA/cm²] Δ Sample 3A 160 1.9 47 Sample 3B 0 4 20

Compared to the sample 3B, in the sample 3A in which the concentrationof B of the memory layer 17 is modulated, Jc0 was diminished and Δ wasimproved. In addition, the coercive force was largely increased in thesample A.

In addition, the coercive force of the sample 3B was 0, but this wasbecause of an easy axis being provided in an in-plane direction of thefilm. That is, the sample 3A in which the concentration of B wasmodulated in the film face direction became the perpendicularmagnetization film, and the sample 3B in which the concentration of Bwas uniform became the in-plane magnetization film.

When comparing the magnetic layer 17 of the samples 3A and 3B, theaverage compositions thereof are the same. However, it was possible toobtain perpendicular magnetic anisotropy by modulating the concentrationof B. The reason for this is considered to be because of the following.

The perpendicular magnetic anisotropy in the sample 3A is an interfacemagnetic anisotropy occurred at an interface between the Co—Fe—B and theMgO insulating layer 16, and this is a phenomenon caused by theoccurrence of a hybrid orbital due to the combination of Co or Fe withoxygen in MgO.

In regard to the sample 3A, it is considered that since theconcentration of B at the interface between the Co—Fe—B and the MgO issmall, the possibility increases that the Co or Fe is combined withoxygen in the MgO, and thereby interface magnetic anisotropy becomesstronger.

In addition, in regard to the sample 3A, the concentration of B of thecap layer 18 side increases, such that the saturation magnetizationdecreases compared to the sample 3B. Therefore, the diamagnetic fieldcomponent decreases, and thereby perpendicular magnetic anisotropy isenhanced.

For this reason, in regard to the sample 3A, it is considered that thecoercive force in the direction perpendicular to the film face, andalong with this, Δ is improved.

In addition, it is considered that the decrease in Jc0 in the sample 3Areflects the phenomenon where the inversion current of the perpendicularmagnetization film becomes lower than that of the in-plane magnetizationfilm.

In addition, in regard to the Experiment 5, the sample A in which theconcentration of B was modulated by four steps was used, but thismodulation may be performed by an arbitrary number of steps. That is,the number of the Co—Fe—B film in which the concentration is changed isnot limited.

In addition, the concentration of B may be continuously changed. Forexample, as shown in FIG. 7, the concentration of B is made to bechanged from the insulating layer 16 to the cap layer 18 in the memorylayer 17. This continuous modulation in the concentration of B may berealized, for example, by the selection of the cap layer, or byadjusting a manufacturing process (heat process or the like).

In addition, the concentration of B may be set in another way, at leastas long as a state after the final manufacturing process of the memoryelement 3 is completed, the concentration of B is low in the vicinity ofthe interface with the insulating layer 16, and as it recedes from theinterface, the concentration of B continuously increases.

As can be seen from the results of this Experiment 5, when thecomposition of the Co—Fe—B magnetic layer making up the memory layer 17is set in such a manner that the concentration of B is made to be low atthe interface with the insulating layer 16 that is an oxide film and theconcentration of B is made to be high at the cap layer 18 side, magneticanisotropy is enhanced.

This is because a Co—O combination or a Fe—O combination that isregarded as an origin of the interface perpendicular magnetic anisotropyis difficult for B to hinder. In addition, the concentration of Bincreases as it recedes from the interface, such that the saturatedmagnetization decreases, and therefore perpendicular magnetic anisotropyis further enhanced.

In this manner, it becomes more advantageous in regard to the decreasein the write current and the improvement in thermal stability.

It should be understood that various changes and modifications to thepresently preferred embodiments described herein will be apparent tothose skilled in the art. Such changes and modifications can be madewithout departing from the spirit and scope and without diminishing itsintended advantages. It is therefore intended that such changes andmodifications be covered by the appended claims.

The application is claimed as follows:
 1. A memory element, comprising: a memory layer that has magnetization perpendicular to a film face and a magnetization direction thereof varies corresponding to information; a magnetization-fixed layer that has magnetization that is perpendicular to the film face and becomes a reference for the information stored in the memory layer; and an insulating layer that is provided between the memory layer and the magnetization-fixed layer and is formed of a non-magnetic layer, wherein an electron that is spin-polarized is injected in a lamination direction of a layered structure having the memory layer, the insulating layer, and the magnetization-fixed layer, and thereby the magnetization direction of the memory layer varies and a recording of information is performed with respect to the memory layer, a magnitude of an effective diamagnetic field which the memory layer receives is smaller than a saturated magnetization amount of the memory layer, the insulating layer is formed of an oxide film, and the memory layer is formed of Co—Fe—B, a concentration of B is low in the vicinity of an interface with the insulating layer, and the concentration of B increases as it recedes from the insulating layer.
 2. The memory element according to claim 1, wherein the insulating layer is formed of MgO.
 3. The memory element according to claim 2, wherein the memory layer has a laminated structure of a plurality of Co—Fe—B layers, and each of the Co—Fe—B layers is configured in such a manner that a layer that comes into contact with the insulating layer is set as a layer having the lowest concentration of B, and as it recedes from the insulating layer, the concentration of B increases.
 4. The memory element according to claim 2, wherein the memory layer is configured in such a manner that at least in a state after the final manufacturing process is completed, the concentration of B is low in the vicinity of the interface with the insulating layer, and as it recedes from the interface, the concentration of B continuously increases.
 5. A memory device, comprising: a memory element that retains information through a magnetization state of a magnetic material; and two kinds of interconnects that intersect each other, wherein the memory element includes, a memory layer that has magnetization perpendicular to a film face and a magnetization direction thereof varies corresponding to information, a magnetization-fixed layer that has magnetization that is perpendicular to the film face and becomes a reference for the information stored in the memory layer, and an insulating layer that is provided between the memory layer and the magnetization-fixed layer and is formed of a non-magnetic layer, an electron that is spin-polarized is injected in a lamination direction of a layered structure having the memory layer, the insulating layer, and the magnetization-fixed layer, and thereby the magnetization direction of the memory layer varies and a recording of information is performed with respect to the memory layer, a magnitude of an effective diamagnetic field which the memory layer receives is smaller than a saturated magnetization amount of the memory layer, the insulating layer is formed of an oxide film, and the memory layer is formed of Co—Fe—B, a concentration of B is low in the vicinity of an interface with the insulating layer, and the concentration of B increases as it recedes from the insulating layer, the memory element is disposed between the two kinds of interconnects, and a current flows to the memory element in the lamination direction through the two kinds of interconnects, and thereby a spin-polarized electron is injected into the memory element. 